A conventional power source apparatus, having a plurality of power source circuits, achieves compactness and consumes less-power by using a common oscillator circuit. One exemplary power supply apparatus is described in Japanese Laid-Open Patent Application Publication No. 2000-11644.
In a typical example, a step-up circuit and a step-down circuit, which operate with a common oscillator circuit, output two different voltages from a common direct current power source. The step-up and step-down circuits may operate simultaneously to consume large power, causing noise in the direct current power source. In such a system, it is difficult to apply the use of common clock signal to items such as cellular phones because excessive noise is generated in the power source.